I have replaced the 02 sensor (the one you can see between the radiator and motor) but my check engine light came back on. I got a PO420 code on it. It says Catalyst System Efficiency Below Threshold ( Bank 1 ). Am I going to have to replace the catalytic converter? And which one? This is on a 1997 ES 300.
Thanks

 

I replaced my oxygen sensor before the cat. I had the same code as u. I have no more check engine light, so try that and see if that helps.

 

Bank 1 is the one facing the firewall. Check for exhaust leaks, make sure post o2 sensor is not faulty, lastly would be a bad cat.